It's long been known that vocabulary and other markers of "crystallised intelligence" continue to improve at least into midlife, but the new findings challenge the long-received wisdom in psychology that abilities related to "fluid intelligence" or thinking on your feet, peak at around age 20 and decline inexorably thereafter.

Joshua Hartshorne and Laura Germine had an advantage over prior researchers in that they sampled people online via the popular websites gameswithwords.org and testmybrain.org, allowing them to gather data from tens of thousands of participants. The brief tests included looking at an array of four unusual shapes and then judging from memory whether a fifth shape matched any of the previous four (a measure of visual working memory); learning symbol and digit pairings (known as "digit symbol coding") or judging a person's emotional expression purely from their eyes.

Analysing performance by age, the researchers discovered that the ability to identify other people's emotions peaks between the ages of 40 to 60 (i.e. there is a long plateau in peak ability); vocabulary ability peaks in the late 60s / early 70s; performance at digit symbol coding (seen as a basic measure of mental agility) peaked at 18; visual working memory peaked at 25; and working memory for numbers peaked in the early to mid-30s.

"At any given age, you’re getting better at some things, you’re getting worse at some other things, and you’re at a plateau at some other things," Hartshorne told MIT News. "There’s probably not one age at which you’re peak on most things, much less all of them."

An important detail for psychologists working in this area is that the study revealed a good deal of consistency between the new data gathered online and comparison data collected 20 years ago with pencil and paper tests, in the development of the Wechsler intelligence tests. This validates the use of the internet to obtain massive samples of data on people's cognitive abilities.

There was one exception – the new data suggest that vocab peaks about 15 years later than the Wechsler data. To follow up on this anomaly, Hartshorne and Germine looked at vocab performance data collected repeatedly from the 1970s to 2012 from thousands of people in paper and pencil tests as part of a large social survey in the US. This showed that over time, people's peak vocab performance is occurring later in life (hence the mismatch of the new internet data with the old Wechsler results). The researchers speculate this may be related to the Flynn effect, and to the fact that, in modern times, people are exposed to more intellectual stimulation later into their lives.

The new findings have important clinical implications. For instance, they suggest that tests for normal functioning in people who are suspected of having neurological illness or damage will need to be sensitive to the trajectories of performance through the lifespan for different mental abilities. And there are theoretical implications too. "The complexities described in this article provide a rich, challenging set of phenomena for theories of development, maturation and ageing," the researchers said.
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Hartshorne, J., & Germine, L. (2015). When Does Cognitive Functioning Peak? The Asynchronous Rise and Fall of Different Cognitive Abilities Across the Life Span Psychological Science DOI: 10.1177/0956797614567339